Synthesis method for carbon material based on lixm1-ym&#39;(xo4)n

ABSTRACT

Method of synthesis for a material made of particles having a core and a coating and/or being connected to each other by carbon cross-linking, the core of these particles containing at least one compound of formula Li x M 1−y M′ y (XO 4 ) n , in which x,y and n are numbers such as 0≦x≦2, 0≦y≦0.6 and 1≦n≦1.5, M is a transition metal, M′ is an element with fixed valency, and the synthesis is carried out by reaction and bringing into equilibrium the mixture of precursors, with a reducing gaseous atmosphere, in such a way as to bring the transition metal or metals to the desired valency level, the synthesis being carried out in the presence of a source of carbon called carbon conductor, which is subjected to pyrolysis. The materials obtained have excellent electrical conductivity as well as very improved chemical activity.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for preparing electrodematerials that are able to make possible redox reactions by exchange ofalkaline ions and electrons. The applications are in the area of primaryor secondary electrochemical generators (batteries), supercapacitygenerators and in the area of modulation systems for electrochromiclight.

PRIOR ART

[0002] Insertion compounds of the formula LiMPO₄ with olivine structure,where M is a metallic cation belonging to the first period [of theperiodic table] of transition metals, e.g. Mn, Fe, Co or Ni, are knownand their use as cathode material in lithium batteries has been reportedby Goodenough et al. in the patent U.S. Pat. No. 5,910,382. In theCanadian patent application with the number CA-A-2,307,119, the generalnature of the “LiMPO₄ type” compounds was indicated insofar as, whileessentially maintaining the same olivine structure, part of the M atomsmay be substituted with other metals with valency between 2 and 3, inwhich the adjacent transition elements, or a part of the phosphorus, canbe substituted by elements such as Si, S, Al, As. Similarly, the lithiumthat allows electroneutrality can occupy a fraction or all of theoctahedral sites of the olivine structure, or possibly position itselfin an interstitial position when all of the octahedral sites areoccupied.

[0003] The formulaLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[SiO₄]_(s)[VO₄]in which:

[0004] M can be Fe²⁺ or Mn²⁺ or a mixture of the two;

[0005] D can be a metal in the +2 oxidation state chosen from the groupcontaining Mg₂₊, Ni²⁺, Co²⁺, Zn²⁺, Cu²⁺ and Ti²⁺;

[0006] T can be a metal in the +3 oxidation state chosen from the groupcontaining Al³⁺, Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Ga²⁺ and V³⁺;

[0007] Q is a metal in the +4 oxidation state chosen from the groupcontaining Ti⁴⁺, Ge⁴⁺, Sn⁴⁺ and V⁴⁺; and

[0008] R is a metal in the +5 oxidation state chosen from the groupcontaining V⁵⁺, Nb⁵⁺ and Ta⁵⁺,

[0009] with a definition of the values taken by parameters x, y, d, t,q, r, p, s and v, encompasses the general nature of the meaning given tothe term “of the Li_(x)MXO₄ type, 0≦x≦2” with olivine structure in themeaning of the present invention and will be used in the following. Thepreferred substituents for the phosphorus are silicon and sulfur.

[0010] In these compounds prepared in the lithiated form (in dischargedstate), at least one of the transition metals is in oxidation state II.In the patent U.S. Pat. No. 5,910,382 and its CIP, as well as in thefollowing patents and publications, the syntheses of the LiMPO₄compounds are all carried out using a salt of the transition metal Mcorresponding to oxidation state II and maintaining this oxidation statethroughout the synthesis, up to the final product. The transitionelement, for which the valency II is maintained throughout the course ofsynthesis, no matter what method is used, is iron, with the majority ofits compounds oxidizing spontaneously. For example, in air, LiFePO⁴ hasbeen produced by reaction in the solid state, at high temperature andunder inert atmosphere, of various constituents (e.g. for the ironsource Fe(OOCCH₃)₂, for the phosphate source, NH₄H₂PO₄ and for that oflithium, Li₂CO₃). In all these cases, the iron source is a salt in whichthe iron is in oxidation state II, which could be using iron (II)acetate as described in the parent U.S. Pat. No. 5,910,382, iron (II)oxalate as described in Electrochem and Solid-State Letters, 3, 66(2000) and in the Proceedings of the 10th IMLB, Como, Italy, May (2000)or vianite (Fe₃(PO₄)₂ 8H₂O) as described in the Canadian patentapplication CA-A-2,270,771. The sensitivity of iron (II) with respect tooxidation by oxygen makes all of these synthesis processes very delicateand all possible precautions must be taken to completely exclude thepresence of oxygen, and in particular at the time of thermal processing,which increases the cost of the material accordingly. This sensitivitygives rise to a lack of reproducibility of the electrochemical behaviorof the samples. This problem is emphasized in Yamada et al., J.Electrochem Soc., 148, A224 (2001). In addition, iron is the most usefulelement which, due to its abundance and lack of toxicity, and theprinciple used in the invention, is intended for an improved preparationof redox compounds containing this element. It is obvious that theresults of the invention apply to manganese, vanadium, cobalt, titanium,vanadium, etc. under corresponding conditions, at their desired degreeof oxidation. In a general way, the precursor of the metal M that isless costly or easier to manipulate does not correspond to the samestate of oxidation as that required in the redox material formula.

[0011] An improvement in these compounds has previously been suggestedin the Canadian patent CA-A-2,270,771. In this document, it has beenshown that the electrochemical performance of LiFePO₄ was greatlyimproved, no matter whether in terms of reversible capacity, cyclabilityor power, when the particles of the material are covered with a finelayer of electronically conductive carbon. In this application, theinventors have benefited from using an iron salt at oxidation state II,in the presence of an organic compound that can be pyrolyzed under thesynthesis conditions without it being possible for the carbon residue tobecome oxidized due to the low oxidizing power of the ferrous compoundor of the atmosphere in equilibrium with it. The patent applicationEP-A-1,094,532 describes a production method for materials for an activepositive electrode. This method includes a step where a number ofsubstances are mixed to obtain a precursor. Then the precursor issintered to result in the synthesis of a compound of the fonnulaLi_(x)M_(y)PO₄, in which x is greater than 0 and less than or equal to2, y is greater than or equal to 0.8 and less than or equal to 1.2 and Mincludes at least one metal having 3d orbitals. A solid reducing agentis added in the course of the mixing step of the precursor in order toallow the preparation, which is carried out under inert atmosphere, ofmaterial for active positive electrodes that are capable of doping anddedoping lithium in a satisfactory and reversible manner.

[0012] EP-A-1,094,533 describes a non-aqueous electrolyte adapted forsecondary batteries using a material or an active electrode containing acompound represented by the general formula Li_(x)M_(y)PO₄, in which xis greater than 0 and less than or equal to 2, and y is greater than orequal to 0.8 and less than or equal to 1.2, with M containing a 3dtransition state, and the grains of Li_(x)M_(y)PO₄ are no greater insize than 10 micrometers. This non-aqueous electrolyte for secondarybatteries is presented as having improved cyclic characteristics and ahigh capacity.

[0013] The international PCT application, reference number WO 01/53198,describes a material based on a mixed lithium metal compound thatreleases lithiunm ions by electrochemical interaction. This material isprepared using the necessary precursors by reduction of at least one ofthe metallic ions by carbon.

[0014] Besides their electrochemical performance in lithium batteries,the interest in this new family of materials is to make use of elementsthat are non-toxic, abundant and inexpensive to extract. Thesecharacteristics are critical to the development of large lithiumbatteries that can be used, in particular, in the electric vehiclemarket where a pressing need has developed in view of the accumulationof greenhouse gases in the environment.

[0015] Thus there is the need to develop a new procedure that is simplerand more reproducible, and less difficult than those that are alreadyknown while still offering improved performance.

SUMMARY OF THE INVENTION

[0016] The present invention describes a synthesis procedure forcompounds of the formula Li_(x)M_(1−y)M′_(y)(XO₄)_(n), by bringing intoequilibrium, in the required proportions, a mixture containingprecursors of the constituents of the compound, and reduction of theequilibrated mixture of the precursors with a reducing gas atmosphere.The initial mixture can be supplemented with a source of carbon, whichmakes possible the preparation of compounds of the formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n) in the form of a material made up ofcarbon-coated grains. The material thus obtained has excellentconductivity.

[0017] These materials can especially be used for the preparation ofelectrochemical cells having an electrolyte and at least two electrodes,of which at least one comprises at least one material synthesizedaccording to one of the procedures according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Legends for the figures cited in the examples

[0019]FIG. 1: 1^(st) cycle obtained by slow voltametry (v=20 mV.h⁻¹) at80° C. for a battery containing non-carbonated LiFePO₄, synthesizedusing FePO₄.2H₂O (reduction by hydrogen) (solid lines) compared to thesame sample after carbonating (dotted lines).

[0020]FIG. 2: Morphology of carbonated LifePO₄ synthesized usingFePO₄.2H₂O (reduction by hydrogen). Micrograph taken on a scanningelectron microscope with 5000× magnification.

[0021]FIG. 3: 5th cycle obtained by slow voltametry (v=20 mV.h⁻¹) at 80°C. of a battery containing carbonated LiFePO₄, synthesized usingFePO₄.2H₂O (reduction by hydrogen) (solid lines) compared to an LiFePO₄obtained using classical synthesis followed by a carbon deposition step(dotted lines).

[0022]FIG. 4: Profiles of charging and discharging carried out ingalvanostatic mode at 80° C. and at two charging and discharging speeds(C/8: solid lines and C/2: dotted lines) for batteries containingcarbonated LiFePO₄ synthesized using FePO₄.2H₂O (reduction by hydrogen).

[0023]FIG. 5: Results of the cycling carried out in galvanostatic modeat 80° C. and at two charging and discharging speeds for batteriescontaining carbonated LiFePO₄ synthesized using FePO₄.2H₂O (reduction byhydrogen).

[0024]FIG. 6: 5th cycle obtained by slow voltametry (v=20 mV.h⁻¹) at 80°C. of batteries containing carbonated LiFePO₄ synthesized usingFePO₄.2H₂O (reduction 1:1 CO/CO₂) for samples containing differentcarbon percentages (0.62%: solid lines, 1.13% dotted lines, 1.35% boldlines).

[0025]FIG. 7: 1st cycle (dotted lines) and 10th cycle (solid lines)obtained using slow voltametry (v=20 mV.h⁻¹) at 80° C. of a batterycontaining LiFePO₄ synthesized using FePO₄.2H₂O (reduction by carbon).

[0026]FIG. 8: Trend in the capacity in the course of cycling of abattery containing LiFePO₄ synthesized using FePO_(4.)2H₂O (reduction bycarbon). Results obtained using slow voltametry (v=20 mV.h⁻¹) at 80° C.

[0027]FIG. 9: 1st cycle (dotted lines) and 10th cycle (solid lines)obtained using slow voltametry (v=20 mV.h⁻¹) at 80° C. of a batterycontaining LiFePO₄ synthesized using FePO₄.2H₂O (reduction by celluloseacetate)

[0028]FIG. 10: Charging and discharging profiles carried out ingalvanostatic mode at ambient temperature at a charging and dischargingspeed of C/24 for batteries containing carbonated LiFe_(0.5)Mn_(0.5)PO₄.

[0029]FIG. 11: Transmission electron microscope micrograph showing thecoating and the cross-linking with carbon of the particles of LiFePO₄.

DESCRIPTION OF THE INVENTION

[0030] A first object of the present invention consists of the synthesisof compounds of the formula Li_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, yand n are numbers such as 0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transitionmetal or a mixture of transition metals from the first line of theperiodic table, M′ is an element with fixed valency chosen among Mg²⁺,Ca²⁺, Al³⁺, Zn²⁺ or a combination of these same elements and X is chosenamong S, P and Si,

[0031] by bringing into equilibrium, in the required proportions, amixture containing at least:

[0032] a) a source of M, at least one part of the said transition metalor metals making up M being in an oxidation state greater than that ofthe metal in the final compound Li_(x)M_(1−y)M′_(y)(XO₄)_(n);

[0033] b) a source of an element M′;

[0034] c) a compound that is a source of lithium; and

[0035] d) possibly a compound that is a source of X,

[0036] the sources of the elements M, M′, Li and X being introduced ornot, in whole or in part, in the form of compounds having more than onesource element, and

[0037] the synthesis being carried out by reaction and bringing intoequilibrium, in the proportions required, the mixture (preferablyintimate and/or homogeneous mixture) of the precursors a) to d) and byreduction in such a way as to bring the transition metal or metals tothe desired degree of valency.

[0038] The reduction may be carried out in different ways, by thetechniques used in the production of ceramics and by different reducingagents, based on the chemistry of carbon derivatives, among them CO,hydrocarbons and various organic compounds, hydrogen and ammonia.

[0039] According to a preferred embodiment of the present invention, thereduction of the mixture of precursors a) to d) is carried out with areducing gaseous atmosphere.

[0040] The source of M can also be the source of X and/or the source ofM′ can also be the source of X and/or the source of lithium can also bethe source of X and/or the source of X can also be the source oflithium.

[0041] According to a preferred embodiment of the invention, bringingthe mixture of precursors a) to d) into equilibrium is carried out inthe form of an intimate and/or homogeneous mixture. In the meaning ofthe present invention, intimate mixture is preferably understood as amixture that does not contain aggregates of particles of individualcomponents of the mixture and that has particle sizes preferably lessthan 10 micrometers, preferably less than 5 micrometers. The methodsthat make this implementation possible are known to the person skilledin the art, such as co-grinding in the presence of balls of hardmaterial (such as steel, tungsten carbide, aluminum, zirconium), dry orin the presence of a liquid, i.e. with equipment of the cyclone typewhere the particles are ground by striking against the walls of theequipment or against itself, ultrasonic energy, or even spray drying ofsolutions or suspensions.

[0042] Homogeneous mixture is understood as a mixture in which no localvariation in concentration of one of the components of the mixture isevident from which the fraction of the mixture chosen for sampling wouldbe extracted. Mechanical agitation by the slanting of blades or ofmagnetic stirring bars for suspensions, planetary mixers with cycloidmovement are examples of the methods used; by definition the solutionsrespond to this criterion.

[0043] In the scope of the present invention, the transition metal ormetals is (are) advantageously chosen at least partially in the groupconstituted by iron, manganese, cobalt and nickel, the remainder of thetransition metals preferably being chosen in the group constituted byvanadium, titanium, chromium and copper.

[0044] Advantageously, the compound that is the source of M is in anoxidation state that can vary from 3 to 7.

[0045] According to a preferred embodiment of the invention, the sourcecompound of M is iron (III) oxide or magnetite, manganese dioxide,di-vanadium pentoxide, trivalent iron phosphate, trivalent iron nitrate,trivalent iron sulfate, iron hydroxyplosphate and lithiumhydroxyphosphate or trivalent iron sulfate or nitrate or a mixture ofthe latter.

[0046] According to another preferred embodiment of the procedure, thecompound that is the source of lithium is chosen from the groupconstituted by lithium oxide or lithium hydroxide, lithium carbonate,the neutral phosphate Li₃PO₄ the acid phosphate LiH₂PO₄, theorthosilicates, the metasilicates or the polysilicates of lithium,lithium sulfate, lithium oxalate and lithium acetate or a mixture of thelatter; still more preferably, the compound that is the source oflithium is lithium carbonate of the formula Li₂CO₃.

[0047] According to another advantageous method, the source of X isselected in the group constituted by sulfuric acid, lithium sulfate,phosphoric acid and its esters, the neutral phosphate Li₃PO₄ or the acidphosphate LiH₂PO₄, the monoammonium or diammonium phosphates, trivalentiron phosphate, manganese and ammonium phosphate (NH₄MnPO₄), silica,lithiumn silicates, alkoxysilanes and their partial hydrolysis productsand mixtures of the latter. Still more advantageously, the compound thatis the precursor of X is iron phosphate, preferably iron (III)phosphate, anhydrous or hydrated.

[0048] The procedure according to the invention works particularly wellfor the preparation of one or more of the following compounds, in whichat least one of the lithium derivatives obtained has the formulaLiFePO₄, LiFe_(1−s)Mn_(s)PO₄ wherein 0≦s≦0.9, LiFe_(1−y)Mg_(y)PO₄ andLiFe_(1−y)Ca_(y)PO₄ wherein 0≦y≦0.3, LiFe_(1−s−y)Mn_(s)Mg_(y)PO₄ wherein0≦s≦1 and 0≦y≦0.2, Li_(1+x)FeP_(1−x)Si_(x)O₄ wherein 0≦x≦0.9,Li_(1+x)Fe_(1−s)Mn_(s)P_(1−x)Si_(x)O wherein 0≦s≦1,Li_(1+z)Fe_(1−s−z)Mn_(s)P_(1−z)S_(z)O₄ wherein 0≦s≦1, 0≦z≦0.2,Li_(1+2q)Fe_(1−s−q)Mn_(s)PO₄ wherein 0≦s≦1, and 0≦q≦0.3,Li_(1+r)Fe_(1−s)Mn_(s)(S_(1−r)P_(r)O₄)_(1.5) wherein 0≦r≦1, 0≦s,t≦1 orLi_(0.5+u)Fe_(1−t)Ti_(t)(PO₄)_(1.5) wherein 0≦t≦1, 0≦u≦2.5. The methodaccording to the invention yields improved results when compounds of theformula Li_(x)M_(1−y)M′_(y)(XO₄)_(n) are obtained that have an olivineor Nasicon structure, including the monoclinic form.

[0049] The reduction is obtained by the action of a reducing atmospherechosen in such a way as to reduce the oxidation state of the metallicion M to the level required for the composition of the compound without,however, reducing it to the neutral metallic state. This reducingatmosphere preferably contains hydrogen or a gas that is capable ofgenerating hydrogen under the synthesis conditions, ammonia or asubstance capable of generating ammonia under the synthesis conditionsor carbon monoxide, these gases being used in their pure state or inmixtures and it also being possible to use them in the presence of watervapor and/or in the presence of carbon dioxide and/or in the presence ofa neutral gas (such as nitroqen or argon).

[0050] According to an advantageous embodiment, the reducing atmosphereis made of a mixture of CO/CO₂ or H₂/H₂O, NH₃/H₂O or a mixture of them,generating an oxygen equilibrium pressure less than or equal to thatdetermined by the transition metal at the state of oxidationcorresponding to the precursors introduced to form the compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n), but greater than that corresponding to thereduction of any one of the transition elements present to the metallicstate, insuring the thermodynamic stability ofLi_(x)M_(1−y)M′_(y)(XO₄)_(n) in the reaction mixture, independent of thesynthesis reaction time.

[0051] According to another advantageous embodiment of the invention,the reducing atmosphere is made of a mixture of CO/CO₂, H₂/H₂O, NH₃/H₂Oor a mixture of them, generating an oxygen equilibrium pressure lessthan or equal to that determined by one of the transition metals presentin Li_(x)M_(1−y)M′_(y)(XO₄)_(n), possibly being able to lead to thereduction of at least this transition element to the metallic state, thecompound Li_(x)M_(1−y)M′_(y)(XO₄)_(n), being obtained by controlling thetemperature and the contact time with the gaseous phase; the synthesistemperature preferably being between 200 and 1200° C., still morepreferably between 500 and 800° C. and the time of contact between thereaction mixture and the gaseous phase preferably being between 2minutes and 5 hours and still more preferably between 10 and 60 minutes.This control is implemented more easily at the time of reduction by agaseous phase due to the rapid diffusion of the gas molecules around thegrains. In addition, the nucleation of the metallic phase is slow andthus can be more easily avoided due to the rapid reduction by thegaseous phase.

[0052] The gaseous reducing atmosphere is preferably obtained bydecomposition, in a vacuum or in an inert atmosphere, of an organiccompound or of a mixture of organic compounds containing at leasthydrogen and oxygen, bound chemically, and pyrolysis of which generatescarbon monoxide and/or a mixture of carbon dioxide and monoxide, ofhydrogen and/or a mixture of hydrogen and water vapor that is able tocarry out the reduction that leads to the formation of the compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n).

[0053] The reducing gas atmosphere is preferably obtained by partialoxidation by oxygen or by air, of a hydrocarbon and/or carbon calledsacrificial carbon.

[0054] Preferably, sacrificial carbon is understood as carbon that isintended to carry out a chemical reaction, in particular with thegaseous phase, which in fact serves as a vector for reduction ofprecursors of the material, the carbon thus being intended to beconsumed.

[0055] In the scope of the present invention) the amount of water vaporpreferably corresponds to between 0.1 and 10 molecules, inclusively, ofH₂O per atom of hydrocarbon at an elevated temperature (preferablycomprised between 400 and 1200° C.) that makes possible the formation ofcarbon monoxide or hydrogen or a mixture of carbon monoxide andhydrogen.

[0056] According to an advantageous method, the sacrificial carbon ischosen from the group constituted by natural or artificial graphitecarbon black or acetylene black and coke (preferably from petroleum),the sacrificial carbon preferably being in the form of particles with asize that is preferably less than 15 micrometers, and still morepreferably less than 2 micrometers.

[0057] The quantity of sacrificial carbon is preferably less than oressentially equal to the quantity required to reduce the reactionmixture without allowing residual sacrificial carbon; this quantity ispreferably a carbon atom that is able to combine with an oxygen atom attemperatures greater than 750° C. and it is preferably an atom that cancombine with 2 oxygen atoms at temperatures less than 750° C.

[0058] According to another advantageous embodiment, the synthesis iscarried out with a quantity of sacrificial carbon that is (essentially)equal, mol for mol, to half the quantity of oxygen needed to be removedfrom the mixture of the precursor compounds to obtain the materialLi_(x)M_(1−y)M′_(y)(XO₄)_(n) by reduction, when the reaction is carriedout below 710° C. and equal, mol for mol, to this quantity when thereaction is carried out below this temperature.

[0059] A second object of the present invention is a method of synthesisfor a material comprising a core and a coating and/or being connected toeach other by cross-linking, the said core comprising at least onecompound of the formula Li_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y andn arc numbers such as 0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transitionmetal or a mixture of transition metals from the first period of theperiodic table, M′ is an element with fixed valency chosen from amongMg²⁺, Ca²⁺, Al³⁺, Zn²⁺ and X is chosen from among S, P and Si, and

[0060] the said coating is made of a layer of carbon,

[0061] the said cross-linking is made of carbon (connecting at least twoparticles to each other),

[0062] the said method consists of bringing into equilibrium (preferablyintimate and/or homogeneous), in the proportions required, a mixturecontaining at least

[0063] a) a source of M, at least one part of the said transition metalor metals making up M being in an oxidation state greater than that ofthe metal in the final compound Li_(x)M_(1−y)M′_(y)(XO₄)_(n);

[0064] b) a source of an element M′;

[0065] c) a compound that is a source of lithium, and

[0066] d) possibly a compound that is a source of X;

[0067] the sources of the elements M, M′, Li and X being introduced ornot, in whole or in part, in the form of compounds having more than onesource element, and

[0068] the synthesis being carried out by reaction and bringing intoequilibrium, in the proportions required, the mixture of the precursorsa) to d) with a reducing gaseous atmosphere, in such a way as to bringthe transition metal or metals to the desired degree of valency,

[0069] the synthesis being carried out in the presence of a carbonsource called carbon conductor,

[0070] the synthesis thus leading to the said material, by a pyrolysisstep for the carbon source compound after, or preferably simultaneouslywith, the steps of preparation of the mixture (preferably intimateand/or homogeneous) of the precursors and reduction of the mixtureobtained.

[0071] The carbon present in the material, in the form of coating andcross-linking, adheres intimately to the material and lends to thelatter an electronic conductivity that is greater than that of thematerial constituted by the corresponding non-coated particles.

[0072] According to a preferred method, the addition of carbon conductoris carried out after the synthesis of the Li_(x)M_(1−y)M′_(y)(XO₄)_(n).

[0073] According to another advantageous embodiment of the invention,the addition of carbon conductor is carried out simultaneously With thesynthesis of the Li_(x)M_(1−y)M′_(y)(XO₄)_(n).

[0074] The reaction parameters, in particular the kinetics of thereduction by the gaseous phase, are chosen in such a way that the carbonconductor does not participate in a significant way in the reductionprocess.

[0075] According to another important variation, the reaction parametersof the synthesis, such as flow and composition of the gaseous phase,temperature and contact time, are chosen in such a way that the carbonconductor does not participate in a significant way in the reductionprocess, i.e. the reduction process is due to the gaseous phase, and inparticular in such a way that the reaction temperature is preferablyless than 900° C. and the reaction time less than 5 hours, in a mannerthat is even more advantageous if the reaction temperature is below 800°C. and/or for a time less than 1 hour.

[0076] According to another variation of the synthesis, the value of xin Li_(x)M_(1−y)M′_(y)(XO₄)_(n) is chosen in such a way as to insurethermodynamic control and/or rapid kinetics of the reduction by makingit possible to select reducing gaseous atmospheres that are easilyaccessible by simple mixture of eases or by reforming simple organicmolecules.

[0077] The organic substance that is the source of carbon conductor isselected in such a way that the particles of material obtained after thepyrolysis step essentially have the form and granulometric distributionof the precursors of the synthesis reaction.

[0078] Thus, the organic substance that is the source of the carbonconductor is advantageously selected from the group constituted bypolymers and oligomers containing a carbon skeleton, simplecarbohydrates or polymers and the aromatic hydrocarbons.

[0079] The organic substance that is the source of carbon conductor ischosen in such a way as to leave a deposit of carbon conductor on thesurface (coating) of the solid particles that are made up of thematerial and/or between these solid particles making up the carbonbridges (cross-linking) at the time of pyrolysis.

[0080] According to another variation, the carbon conductor sourcecontains, in the same compound or in the mixture that constitutes thissource, oxygen and hydrogen that are bound chemically and from whichpyrolysis locally releases carbon monoxide and/or carbon dioxide and/orhydrogen and water vapor that contributes, in addition to depositing,carbon, to creating locally the reducing atmosphere required forsynthesis of the material Li_(x)M_(1−y)M′_(y)(XO₄)_(n).

[0081] Thus, the organic substance that is the source of carbonconductor source is at least one of the compounds of the groupconstituted by polyethylene, polypropylene, glucose, fructose, sucrose,xylose, sorbose, starch, cellulose and its esters, block polymers ofethylene and ethylene oxide and polymers of furfuryl alcohol.

[0082] The source of carbon conductor is preferably added at the startof, or in the course of, the mixing step of the reaction precursors a)to d).

[0083] According to a preferred method, the amount of substance that isthe carbon conductor source, present in the reaction medium subjected toreduction, is chosen such that the amount of carbon conductor in thereaction mixture is preferably comprised between 0.1 and 25%,inclusively, and still more preferably comprised between 0.3 and 1.5%,inclusively, of the total mass of the reaction mixture.

[0084] Advantageously, in the method according to the first object ofthe invention, as well as in the method according to the second objectof the present intention, the thermal processing (which includes theformation reaction of Li_(x)M_(1−y)M′_(y)(XO₄)_(n) and the reduction andpyrolysis) is carried out by heating from normal temperature to atemperature comprised between 500 and 1100° C. in the presence of areducing atmosphere such as defined above. This maximum temperaturereached is even more advantageously comprised between 500 and 800° C.

[0085] According to another advantageous embodiment, the temperature andduration of the synthesis are chosen as a function of the nature of thetransition metal, i.e. above a minimum temperature at which the reactiveatmosphere is capable of reducing the transition element or elements totheir oxidation state required in the compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n) and below a temperature or a time leadingto a reduction of the transition element or elements to the metallicstate or an oxidation of the carbon resulting from pyrolysis of theorganic substance.

[0086] According to another advantageous embodiment of the procedureaccording to the second object of the present invention, the heart ofthe core of the particles of the core synthesized is at least 95% anLi_(x)M_(1−y)M′_(y)(XO₄)_(n) compound (preferably the compoundsynthesized has the formula LiMPO₄), the remainder may be an oxide ofone or [more] of the metals of the precursors, functioning as aninsertion or inert compound, carbon, carbonate or lithium phosphate andthe amount of carbon conductor after pyrolysis is comprised between 0.1and 10% by mass in comparison to the mass of the compound LiMPO₄.

[0087] The compound that is the source of carbon conductor isadvantageously chosen such that it is easily dispersible at the time ofmixture with the precursors. The intimate and/or homogeneous mixture ofprecursors a) to d) is advantageously produced by agitation and/or bymechanical grinding and/or by ultrasonic homogenizing, in the presence,or not, of a liquid or by spray-drying of a solution of one or moreprecursors and/or of a suspension and/or of an emulsion.

[0088] According to a particularly advantageous embodiment the synthesisaccording to the present invention comprise the two steps:

[0089] i) intimate grinding dry or in a solvent, of the source compoundsincluding carbon, and drying if necessary; and

[0090] ii) thermal processing with scavenging by a controlled reducingatmosphere.

[0091] The procedures according to the invention make it possible tocarry out the synthesis of materials having a conductivity that isgreater than 10⁻⁸ Scm⁻¹, measured on a sample of powder compacted at apressure greater than or equal to 3000, preferably 3750 Kg.cm⁻².

[0092] The conductivity measurement is carried out on powders of thesample. This powder (from 100 mg to around 1 g) is placed in a hollowcylindrical mold, 1.3 cm in diameter, made of poly(oxymethylene)(Delrin®) and it is compacted between two stainless steel pistons with alaboratory press having a force of 5.10³ Kg, which corresponds to apressure of 3750 Kg.cm⁻².

[0093] The conductivity measurement is carried out by using the pistons(plungers) as electrodes and using the complex impedance method known tothe person skilled in the art. The conductivity is obtained from theresistance, using the formula ρ=RS/l where R is the measured resistance,S is the surface (1.33 cm² for 1.3 cm diameter), l is the thickness ofthe sample and the resistivity is determined using the formula ρ=RS/l.

[0094] One of the compounds advantageously prepared by the procedureaccording to the first object of the present invention is the compoundof formula LiFePO₄.

[0095] One of the materials advantageously obtained by the synthesisprocedure according to the second object of the present invention ismade up of particles having a core, a coating and/or a cross-linking.The core of the particles is essentially (preferably at least 95%) madeup of a compound of formula LiFePO₄, the remainder being made up ofother compounds, in particular other oxides having an activity or not,or lithium phosphate or ferric phosphate and in which the coating and/orcross-linking of the particles of the material by carbon preferablyrepresents an amount of carbon conductor between 0.2 and 5%, preferablybetween 0.3 and 3%, in comparison to the total mass of the materialobtained.

[0096] The compound that is the source of iron, in particular thecompound that is the source of iron in the synthesis of the compound offormula LiFePO₄, is chosen at least partially from the group constitutedby iron phosphates, iron oxyphosphates or hydroxyphosphates, iron oxidesand lithium oxides, in which at least a part of the iron is in theoxidation state III, as well as mixtures of the latter.

[0097] The compound that is the source of lithium is advantageouslylithium phosphate, lithium dihydrogenophosphate, lithium carbonate,lithium acetate or lithium hydroxide, as well as mixtures of the latter.

[0098] The compound that is the source of phosphorus is advantageouslyammonium phosphate, orthophosphoric, metaphosphoric or pyrophosphoricacid or phosphorus pentoxide.

[0099] The synthesis procedure according to the invention can be carriedout in the reactor that is used, or was used, for the preparation of themixture of precursors or in a different reactor (preferably in areformer).

[0100] A third object of the present invention is made up by particlesof a compound of formula Li_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y andn are numbers such as 0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transitionmetal or a mixture of transition metals from the first line of theperiodic table, M′ is an element with fixed valency chosen from amongMg²⁺, Ca²⁺, Al³⁺, Zn²⁺ and X is chosen from among S, P and Si, the saidcompound having a conductivity greater than 10⁻⁸ S.cm⁻¹, measured on asample of powder compressed at a pressure of 7350 Kg.cm², the particleshaving a size between 0.05 micrometers and 15 micrometers, preferablybetween 0.1 and 10 micrometers.

[0101] A fourth object of the present invention consists of a materialthat can be obtained by a procedure according to the second object ofthe present invention, comprising a core and a coating and/or across-linking, said material having a total carbon amount greater than0.1%, preferably between 0.1 and 0.2%, of the total mass of thematerial.

[0102] According to an advantageous embodiment of this fourth object,the said core contains at least one compound of the formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals from the first line of the periodic table, M′ is anelement with fixed valency chosen from among Mg²⁺, Ca²⁺, Al³⁺, Zn²⁺ andX is chosen from among S, P and Si, the said material having aconductivity greater than 10⁻⁸ S.cm⁻¹, measured on a sample of powdercompacted at a pressure of 3750 Kg.cm⁻².

[0103] A fifth object of the present invention consists of a materialhaving a core and a coating and/or a cross-linking, the said corecomprising at least one compound of formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals from the first line of the periodic table, M′ is anelement with fixed valency chosen in the group constituted by Mg²⁺,Ca²⁺, Al³⁺, Zn²⁺ and X is in the group constituted by S, P and Si, thesaid material having a conductivity greater than 10⁻⁸ S.cm⁻¹, measuredon a sample of powder compacted at a pressure of 3750. The materialsthus obtained have, according to the measurement method explained above,good conductivity that in some cases is greater than 10⁻⁸ Scm³¹ ¹ on asample of compacted powder and one of carbon greater than 0.1%,preferably between 0.1 and 0.2%, of the total mass of the material.

[0104] The uses of these materials are very important in the area ofelectrochemistry, as electrode material, used alone or in a mixture withother electrically active materials, in particular in cells serving asprimary or secondary generators, possibly connected in batteries oraccumulators; in super-capacities, systems capable of storing electricalenergy with significant power (≧800 Wl³¹ ¹), in electrochromic lightmodulation systems and antiglare mirrors for automobiles. In systems forseparation or purification of metals, especially of lithium, waterpurification, in oxidation reactions or reduction reactions in organicsynthesis; in the case of oxidation reaction it may be necessary todelithiate the material chemically or electrochemically to increase itsoxidizing power.

[0105] A sixth object of the present invention consists ofelectrochemical cells containing at least two electrodes and at leastone electrolyte, these cells being characterized in that at least one oftheir electrodes comprises at least one compound according to the thirdobject of the invention.

[0106] A seventh object of the present invention consists ofelectrochemical cells containing at least two electrodes and at leastone electrolyte, these cells being characterized in that at least one oftheir electrodes comprises at least one material according to the fourthobject of the invention.

[0107] These cells are preferably designed in such a way that theelectrolyte is a polymer, solvating or not, optionally plasticized orgelled by a polar liquid containing one or more metallic salts insolution, by way of example.

[0108] Advantageously, the electrolyte is a polar liquid immobilized ina microporous separator, containing one or more metallic salts insolution; by way of example at least one of these metallic salts is alithium salt.

[0109] Preferably at least one of the negative electrodes is made ofmetallic lithium, a lithium alloy, especially with aluminum, antimony,zinc, tin, possibly in nanomolecular mixture with lithium oxide or acarbon insertion compound, especially graphite, a double nitride oflithium and iron, cobalt or manganese, a lithium titanate of the formulaLi_(x)Ti_((5+3y)/4)O₄, wherein 1≦x≦(11−3y)/4 (or) wherein 0≦y≦1.

[0110] According to another embodiment of the cells according to theinvention, at least one of the positive electrodes contains one of theproducts that can be obtained by a procedure according to the invention,used alone or in a mixture with a double oxide of cobalt and lithium orwith a complex oxide of the formulaLi_(x)Ni_(1−y−z−q−r)Co_(y)Mg_(z)Al_(r)O₂ wherein 0.1≦x≦1, 0≦y, z andr≦0.3or with a complex oxide of the formulaLi_(x)Mn_(1−y−z−q−r)Co_(y)Mg_(z)AlrO₂−qF_(q) wherein 0.05≦x≦1 and 0≦y,z, r, q≦0.3.

[0111] The polymer used to bond the electrodes or used as electrolytesis advantageously a polyether, a polyester, a polymer based on methylmethacrylate units, an acrylonitrile-based polymer and/or a vinylidenefluoride, or a mixture of the latter.

[0112] Preferably, the cell contains a solvent that is preferably anon-protogenic solvent that contains, e.g. ethylene or propylenecarbonate, an alkyl carbonate having 1 to 4 carbon atoms,γ-butyrolactone, a tetraalkylsulfamide, an α-ω dialkyl ether of a mono-,di-, tri-, tetra- or oligo-ethylene glycol with molecular weight lessthan or equal to 5000, as well as mixtures of the above-named solvents.

[0113] The cells according to the invention preferably function asprimary or secondary generator, as supercapacity or as light modulationsystem.

[0114] According to another preferred method, the electrochemical cellsaccording to the present invention function as supercapacity,characterized in that the positive electrode material is a materialaccording to the third, fourth or the fifth object of the presentinvention and the negative electrode is a carbon with a specific surfacearea greater than 50 m².g⁻¹ in the form of powder, fiber or mesoporouscomposite of the carbon-carbon composite type.

[0115] According to another embodiment, the electrochemical cellsfunction as a light modulation system and in that the optically inactivecounter-electrode is a material according to the third, fourth and fifthobject of the present invention, spread in a thin layer on a transparentconductor support of a glass or polymer type covered with doped tinoxide (SnO₂:Sb or SnO₂:F) or doped indium oxide (In₂O₃:Sn).

[0116] Preferred Methods

[0117] The proposed invention relates to a new method for simplifiedsynthesis of Li_(x)MXO₄ compounds with olivine structure obtained byreduction of a mixture in which at least a part of the transition metalM is in an oxidation state higher than that of the final compoundLiMPO₄. Another surprising advantage of the present invention is to alsobe compatible with the synthesis described in CA-A-2,270,771, whichleads to optimized performance. In this case, the organic compound thatis the carbon source is added to the mixture of the initial reagentscontaining, at least partially, transition metal in a state of oxidationgreater than that of the lithium compound LiMPO₄ and the simplifiedsynthesis leads directly to the material covered in carbon. Thesimplification involves, in particular, a reduction in the number ofsteps and above all, in the number of steps where control of theatmosphere is necessary. Reference can be made to “Modern Batteries”, byC. A. Vincent & B. Scrosati, Arnold publishers, London, Sydney,Auckland, (1997).

[0118] The improvements also relate to the reproducibility of thesynthesis, to the control of the size and distribution of the particlesand to a reduction in the number and cost of the initial reagents andnaturally of the final material. This synthesis, when combined with theteachings of CA-A-2,270,771, also makes it possible to control theamount of carbon in the final material.

[0119] We are reporting here, for the first time, the synthesis of aLi_(x)MXO₄ compound of olivine type, in this case LiFePO₄, produced byreduction of an iron (III) salt. Since the initial salts are no longersensitive to oxidation, the synthesis process is greatly simplified. Inaddition, the possible use of Fe₂O₃ as a source of iron considerablyreduces the cost of synthesizing LiFePO₄. This material would thus bepreferable to other cathode materials for lithium batteries, such ascobalt or nickel oxides in the case of lithium-ion batteries, orvanadium oxides V₂O₅ or analogs that are less inoffensive to theenvironment.

[0120] LiFePO₄ can be prepared using an iron (III) salt that is stablein air, e.g. FePO₄.2H₂O or Fe₂O₃ or any other source of iron (III). Thelithium source would be e.g. Li₂CO₃ in the first case, or LiOH, LiH₂PO₄or Li₃PO₄ would be used as a source of both lithium and phosphorus inthe second case. The stoichiometric mixtures, as well as the carbonprecursor, are processed at 700° C. for 4 hours with scavenging by anexcess of reducing atmosphere in such a way as to reduce the oxidationstate of the iron. The choice of the synthesis atmosphere andtemperature is very important in order to be able to reduce the iron(III) to iron (II) without the gaseous atmosphere or the carbon presentbeing able to reduce the iron to the metallic state. The latter willpreferably, but in a non-limiting manner, be made up e.g. of hydrogen,ammonia, of a Gaseous mixture capable of supplying hydrogen under thesynthesis conditions, the hydrogen being able to be used pure or dilutedin a dry or hydrated inert gas, carbon monoxide, possibly mixed withcarbon dioxide and/or a dry or hydrated neutral gas. The maximum thermalprocessing temperature is chosen such that the carbon present will bethermodynamically stable with respect to the iron (II) and preferablywith respect to the gaseous phase. In the case of iron, the limittemperature zone is between 500 and 800° C., preferably around 700° C.Beyond these temperatures, the carbon becomes sufficiently reducing toreduce the iron (II) to metallic iron. In the case of other transitionmetals., any person skilled in the art would be able to use Ellinghamcurves to adapt the temperature and the nature of the gaseous atmospherein order to obtain an equivalent result.

[0121] An unexpected and surprising aspect of the invention that isadvantageous is the relative chemical inertia of the carbon deposited onthe surface of the material with respect to reactions that make itpossible to reduce the degree of oxidation of the transition metal, inparticular, of iron. From a thermodynamic point of view, the carbonformed by decomposition of the pyrolyzed organic substance has areducing power that is adequate to oxidize into CO₂ or CO and to reduce,even in an inert atmosphere, Iron (III) to Iron (II), which would makecontrolling the amount of carbon in the final product difficult. Theinventors have noted that the reduction reaction was almost totally dueto the action of the reducing gas atmosphere, of which the kinetics arefaster than those due to the action of the carbon deposited on thesurface, in spite of the intimate contact between the two solid phases(carbon and redox material). By using a reducing atmosphere, preferablybased on hydrogen, ammonia or carbon monoxide, the reduction of the ironby the solid carbon is not promoted kinetically and the Iron (III) isreduced to Iron (II) mainly by reaction with the reducing atmosphere.The amount of carbon in the final product thus essentially correspondsto the decomposition yield of the organic substance, which makes itpossible to control this amount.

[0122] The following examples are given to better illustrate the presentinvention, but they should not be interpreted as constituting alimitation to the scope of the present invention.

EXAMPLES Example 1 Synthesis of LiFePO₄ Using Iron Phosphate in ReducingAtmosphere

[0123] LiFePO₄ was prepared by reaction of FePO₄.2H₂O and Li₂CO₃ in thepresence of hydrogen. In a first step, stoichiometric quantities of thetwo compounds are ground together in isopropanol, then heatedprogressively (6° C. per minute up to 700° C.) in a tube kiln underreducing gas scavenging (8% hydrogen in argon). This temperature ismaintained for one hour. The sample is cooled for 40 minutes, whichwould be with a cooling speed of around 15° C. per minute.

[0124] The reducing gas flow is maintained during the entire thermalprocessing time and also during the temperature drop. The total thermalprocessing time is around three and a half hours.

[0125] The structure of the sample was verified by X-ray diffraction andthe rays correspond to those of pure triphylite LiFePO₄.

Example 1′ Preparation of LiFePO₄ Coated with Carbon Synthesized Usingthe Sample Prepared in Example 1

[0126] The triphylite obtained in example 1 is impregnated with asolution of cellulose acetate (39.7% by weight of acetyl, averagemolecular weight M_(w) of 50,000) in acetone. The quantity of celluloseacetate added represents 5% of the weight of the processed triphylite.The use of a carbon precursor in solution makes possible a perfectdistribution over the particles of triphylite. After drying, the mixtureis placed in the kiln described above under scavenging by an argonatmosphere. The temperature is increased by 6° C. per minute up to 700°C. The latter temperature is maintained for one hour. The sample is thencooled progressively, still under argon scavenging. This sample contains1% by weight of carbon, which corresponds to a carbonation yield of thecellulose acetate of 20%.

[0127] The material exhibits electronic surface conductivity. The latterwas measured on a pastille of compacted powder. A force of 5 tons isapplied at the time of measurement on a sample that is 1.3 cm indiameter. Under these conditions, the electronic conductivity measuredis a 5.10⁻⁵ S.cm⁻¹.

Example 1″ Comparison of the Electrochemical Behavior of MaterialsPrepared in Examples 1 and 1′ in Electrochemical Cells

[0128] The materials prepared in example 1 and 1′ were tested inbutton-type CR 2032 cells of lithium polymer batteries at 80° C. Thecathodes were prepared by mixing the powder of the active material withcarbon black (Ketjenblack®) to insure electronic exchange with thecurrent collector and poly(ethylene oxide) with mass 400,000 used as thebinding agent on one hand, and ionic conductor on the other. Theproportions by weight are 51:7:42. Acetonitrile is added to the mixtureto dissolve the poly(ethylene oxide) in a quantity that is adequate toform a homogeneous suspension. This suspension is then dripped onto a 1cm² stainless steel disk. The cathode thus prepared is dried in avacuum, then transferred in a glove box under helium atmosphere (<1 ppmH₂O, O₂). A sheet of lithium (27 μm) laminated on a nickel substrate wasused as the anode. The polymer electrolyte was made of poly(ethyleneoxide) with mass 5,000,000 and a bistrifluorosulfonimide lithium saltLi[(CF₃SO₂)₂N]) (hereinafter referred to as LiTFSI) in oxygenproportions of oxyethylene units/lithium ions of 20:1.

[0129] Electrochemical experiments were carried out at 80° C., thetemperature at which the ionic conductivity of the electrolyte isadequate (2×10⁻³ Scm⁻¹).

[0130]FIG. 1 shows the first cycle obtained by slow voltametry, atechnique well known to the person skilled in the art (20 mV.h⁻¹),controlled by a Macpile® type battery cycler (Biologic™, Claix, France),of the samples prepared in example 1 and 1′.

[0131] The non-carbonated compound in example 1 shows the oxidoreductionpeaks characteristic of LiFePO₄. The capacity exchanged at the time ofthe reduction process represents 74% of the theoretical value. Thereaction kinetics are slow and the discharge extends to 3 volts. Thesecapacity and kinetic limitations of the reactions are currently observedfor the samples of non-carbonated LiFePO₄. The carbonated compound fromexample 1′ shows well-defined oxidoreduction peaks and reaction kineticsthat are much more rapid than those of the material resulting from thesynthesis described in example 1. The capacity achieved in discharge is87% of the theoretical value, which represents an improvement in theelectrochemical generator capacity of 17% in comparison to that of thenon-carbonated sample in example 1.

Example 2 Synthesis of Carbonated LiFePO₄ Using Iron Phosphate inReducing Atmosphere

[0132] Carbonated LiFePO₄ was prepared by reducing reaction ofFePO₄.2H₂O and Li₂CO₃ in the presence of hydrogen. In a first step, thestoichiometric quantities of the two compounds, as well as the carbonsource, (cellulose acetate, 39.7% by weight of acetyl, average molecularweight M_(w) of 50,000) in low proportion (5% by weight in comparison tothe weight of FePO₄ 2H₂O, i.e. 4.2% in comparison to the weight of themixture of FePO₄.2H₂O and Li₂CO₃) are ground together in isopropanol.The solvent is evaporated and the mixture subjected to the thermalprocessing described in examples 1 and 1′. Throughout the entire thermalprocessing and also at the time of the temperature drop, the reducingatmosphere is applied by a scavenging of a mixture of 8% hydrogen inargon.

[0133] The structure of the sample was verified using X-ray diffractionand the rays correspond to those of pure triphylite LiFePO₄.

[0134] The prepared sample is constituted by very fine particles on theorder of a micrometer (FIG. 2). These particles are covered with a finelayer of carbon, of which the weight represents 1.2% of the total weightof the sample, measured by gravimetry after dissolving the core ofLiFePO₄ in 2M hydrochloric acid.

[0135] The material exhibits electronic surface conductivity. The latterwas measured according to the procedure described in example 1′. Underthese conditions, the electronic conductivity measured is 2.10⁻³ Scm⁻¹.

[0136] Taking into account the residual quantity of carbon in thesample, the carbonation yield of the cellulose acetate at the time ofsynthesis is 20%. It is important to note that this yield is identicalto that obtained in example 1′, where the triphylite LiFePO₄ is alreadyformed and no reducing step is necessary.

[0137] Thus it is evident that the carbon that comes from decompositionof the cellulose acetate is not consumed and does not interfere in thereaction that reduces iron (III) to iron (II). Thus this reduction iscarried out by means of the gaseous phase.

Example 2′ Comparison of the Electrochemical Behavior of the CarbonatedTriphylite LiFePO₄ Prepared in Example 1 to That of a Sample ofCarbonated Triphylite Synthesized by Another Method

[0138] The material prepared in example 2 was tested in CR 2032 buttoncells described in example 1′. For comparison, we also are reportingseveral results obtained for the best carbonated sample synthesizedusing iron (II) (vivianite Fe₃(PO₄)₂.8H₂O), the synthesis of which hasalready been described in CA-A-2,270,771.

[0139]FIG. 3 presents the 5^(th) cycle obtained by slow voltametry (20mV.h⁻¹) controlled by a battery cycler of the Macpiles® type with thesample resulting from classical synthesis (dotted lines) on one hand, tothat obtained in example 2 (solid lines) on the other. The two syntheseslead to samples having the same electrochemical behavior on the level ofoxidoreduction potentials and electrochemical kinetics.

[0140] The charging and discharging profiles of batteries assembled withthe sample resulting from the synthesis described in example 2 arepresented in FIG. 4 for two loads. These results are obtained ingalvanostatic mode between 2.8 and 3.8 volts for two charging anddischarging speeds C/8 and C/2 (the current applied (expressed in mA) atthe time of charging or discharging corresponds to ⅛ (or ½ respectively)of the theoretical capacity of the battery expressed in mAh). We havereported the 20^(th) cycle and in the two cases, the discharge plateauis flat and the capacities involved correspond to 95% of the theoreticalcapacity.

[0141] The trend in capacities exchanged at the time of discharging isrepresented in FIG. 5. In both cases, the initial capacity is around 80%of the theoretical capacity but, after around ten cycles, it is greaterthan 95%, ie. at 160 mAh.g⁻¹, and remains stable for the duration of theexperiment. These results are comparable to those obtained withclassical synthesis (reaction of divalent iron phosphate (vivianite)with lithium phosphate).

Example 3 Control of the Carbon Quantity

[0142] Samples of triphylite with different amounts of carbon wereprepared by reaction of FePO₄.2H₂O and Li₂CO₃ in the presence of a 1:1mixture by volume of CO/CO₂. This atmosphere was chosen for its reducingpower with respect to iron (III) while maintaining a stability of theiron (II), in particular at the end of the cycle for the rise to thesynthesis temperature at 700° C. In a first step, the stoichiometricquantities of the two compounds, as well as the cellulose acetate, areground together in isopropanol. The cellulose acetate quantities addedrepresent 2.4 and 5%, respectively, of the mixture weight. After drying,these mixtures are heated progressively (6° C. per minute up to 700° C.)in a tube kiln with scavenging of the reducing gas (CO/CO₂: 1:1). Thistemperature is maintained for one hour. The sample is cooled for 40minutes, which would be with a cooling speed of around 15° C. perminute. The reducing gas flow is maintained during the entire thermalprocessing time and also during the temperature drop. The total thermalprocessing time is around three and a half hours.

[0143] The structure of the samples was verified by X-ray diffractionand in all cases, the rays correspond to those of pure triphyliteLiFePO₄.

[0144] The amounts of carbon were determined by elementary analysis. Theresults, as well as the electronic conductivities, of the samples areshown in Table 1 below. TABLE 1 % Cellulose acetate Amount of C Yield(C) Conductivity 2 0.62 0.22 2.10⁻⁶ S.cm⁻¹ 4 1.13 0.2  1.10⁻³ S cm⁻¹ 51.35 0.19 4.10⁻² S.cm⁻¹

[0145] In the three cases, the carbonation yield (yield (C) of table 1for cellulose acetate) is close to 20%.

[0146] The residual carbon quantity has a significant influence on theelectronic conductivity. As can be seen, the quantities of carbonconductor are proportional to the quantity of precursor added (celluloseacetate). This demonstrates, in a formal way, that the carbon conductordoes not participate in the reduction of iron (III) in the presence ofreducing gas atmosphere, the latter reducing the iron compound with morerapid kinetics.

Example 3′ Comparison of Electrochemical Behavior of the Samples ofCarbonated Triphylite Prepared in Example 3

[0147] The materials prepared in example 3 were tested in CR 2032 buttoncells described in example 1″.

[0148]FIG. 6 shows the 5^(th) cycle obtained by slow voltametry (20mV.h⁻¹) controlled by a battery cycler of the Macpile® type with:

[0149] the sample containing 0.62% carbon (solid lines);

[0150] the sample containing 1.13% carbon (dotted lines); and

[0151] the sample containing 1.35% carbon (bold lines).

[0152] The main characteristics of the electrochemical behavior of thesesamples are summarized in Table 2 below: TABLE 2 % Carbon 0.62 1.13 1.35Capacity (mAh.g⁻¹) 150 160 163 % Theoretical capacity 88 94 96 I peak(mA) 52 60 73

[0153] The residual carbon quantity has an important influence on thecapacity of the samples. In addition, the increase in the peak currentwith the amount of carbon indicates an improvement in the reactionkinetics. The latter reflects the increase in electronic conductivitywith the amount of carbon specified in example 3.

[0154] The synthesis method described in example 3 males it possible toreliably and reproducibly control the amount of carbon in the finalmaterial. This is essential, taking into account the influence of theamount of carbon on the electrochemical properties.

Example 4 Counter-Example of Reduction Using Carbon

[0155] LiFePO₄ was prepared by reaction of FePO₄.2H₂O and Li₂CO₃ in thepresence of carbon, in inert atmosphere according to the proceduredescribed in the PCT application with the number WO 01/53198.

[0156] The stable oxidation product of the carbon is CO₂ below 710° C.and CO above that. In addition, above 400° C., CO₂ reacts on the excessof carbon to form CO. This latter reaction is equilibrated and theCO/CO₂ ratio depends on the temperature. Thus it is difficult todetermine the carbon quantity to be added to the mixture of FePO₄.2H₂Oand Li₂CO₃. If only CO₂ is produced, ¼ mol of carbon is enough to reduceone mol of iron (III) to iron (II) and if only CO is produced, ½ mol ofcarbon is necessary for the same reaction.

[0157] This synthesis was carried out for two different mixturecompositions:

[0158] First mixture (mixture 4A), identical to that in patent WO01/53198

[0159] 1 mol of FePO₄.2H₂O

[0160] ½ mol of Li₂CO₃

[0161] 1 mol of C

[0162] In the case where only CO would be produced at the time of iron(III) reduction, this stoichiometry would correspond to an excess ofcarbon of 100%.

[0163] Second mixture (mixture 4B)

[0164] 1 mol of FePO₄.2H₂O

[0165] ½ mol of Li₂CO₃

[0166] ½ mol of C

[0167] Stoichiometric mixture if only CO is produced at the time of ironreduction. The synthesis procedure used is the one proposed in WO01/53198: the mixtures are ground in isopropanol, then dried. The powderis then compacted into pastilles. The pastilles are placed in a tubekiln with argon scavenging. The temperature of the kiln is broughtprogressively to 750° C. at a heating speed of 2° C. per minute. Thesample is held at 750° C. for 8 hours according to WO 01/53198 before itis cooled at 2° C. per minute to ambient temperature. For the entireduration of thermal processing, the enclosed space in the kiln isscavenged by an argon current. The total time of this thermal processingis 20 hours. The pastilles are then powdered. Elementary analysesindicating that the two samples contain carbon are shown in table 3below: TABLE 3 Initial carbon Final carbon Residual Carbon (Mol) %sample carbon (Mol) consumed Mol Sample A 1 5.87 0.82 0.18 Sample B 0.51.7 0.23 0.27

[0168] Synthesis B indicates that only around ¼ mol of carbon wasconsumed by transformation into CO₂ to reduce one mol of iron (III) toiron (II), even though the final temperature is greater than 710° C.This confirms the difficulty of controlling the stoichiometry by thisreducing method.

[0169] At the time of synthesis of sample A, carried out according tothe teachings of WO 01/53198, the quantity of carbon consumed isinadequate for the reduction reaction of iron (III) to iron (II) to becomplete. By increasing the quantity of carbon, the probability isdecreased of having the triple points of contact between iron phosphate,lithium carbonate and carbon that are necessary for formation ofLiFePO₄.

[0170] At the time of this synthesis, the maximum formation yield ofLiFePO₄ is 80%. Taking into account the residual carbon present, thepurity of sample A is around 75%. This consideration corroborates themediocre electrochemical activity of 70% obtained according to theteachings of WO 01/53198.

Example 4′ Electrochemical Behavior of Sample B Synthesized in Example 4

[0171] The material 4B prepared in example 4 was tested in CR 2032button cells described in example 1″. FIG. 7 shows the first (dottedlines) and the 10^(th) (solid lines) cycles obtained by slowvoltametry(20 mV.h⁻¹) controlled by a Macpile® type battery cycler. FIG.8 illustrates the trend in the battery capacity with cycling.

[0172] These two figures show a rapid deterioration in theelectrochemical behavior of the sample. The kinetics are slower,starting from the 10^(th) cycle. In addition, after 10 cycles, thebattery has lost 23% of its initial capacity. This behavior is generallyobserved for samples of LiFePO₄ not covered with carbon. The residualcarbon, dispersed in the material, does not have the same beneficialeffect as the carbon coating the grains of LiFePO₄ and deriving from thedecomposition of an organic substance.

Example 5 Reduction by the Substances Produced at the Time ofDecomposition of an Organic Substance

[0173] Reduction by cellulose acetate (39.7% by weight of acetylatedgroups)

[0174] In a first step, stoichiometric quantities of the two compoundsFePO₄.2H₂O and Li₂CO₃, as well as cellulose acetate, are ground togetherin acetone. The quantity of cellulose acetate added represents 5% of theweight of the initial mixture.

[0175] For example:

[0176] FePO₄.2H₂O: 186.85 g

[0177] Li₂CO₃: 36.94 g

[0178] Cellulose acetate 11.19 g

[0179] After drying, the mixture is placed in a tube kiln with UHP(Ultra High Purity) argon scavenging; this gas also circulates across anoxygen trap, of which the residual amount is <1 pm). The kiln is heatedprogressively at 6° C. per minute, up to 200° C. This temperature ismaintained for 20 minutes to dehydrate the iron phosphate. Thetemperature is increased at the same heating speed up to 400° C. Thistemperature is maintained for 20 minutes to decompose the celluloseacetate and the lithium carbonate. After a third ramp up to 700° C. at6° C. per minute, where the sample is held for one hour to impart bettercrystallinity, the sample is cooled progressively. The inert gas flow ismaintained during the entire thermal processing time and also during thetemperature reduction.

[0180] The structure of the samples was verified by X-ray diffractionand in all cases, the rays correspond to those of pure triphyliteLiFePO₄.

[0181] The elementary analysis shows that the sample contains 0.74%carbon (0.098 mol of carbon per mol of LiFePO₄).

[0182] The electronic conductivity measured as described in example 1′is 5.10⁻⁴ S.cm⁻¹.

[0183] The carbonation yield of the cellulose acetate given in theliterature is 24%. Using 11.19 g cellulose acetate, it is possible toobtain 2.68 g, i.e. 0.22 mol carbon. At the end of the reaction, 0.098mol carbon remains. The quantity of carbon consumed is not sufficient toexplain the reduction of iron (III) to iron (II) by the carbon.

[0184] To explain the reduction of iron (III) to iron (II), it isnecessary to consider the intervention of a local reducing gaseous phaseresulting from the decomposition of the cellulose acetate.

[0185] The cellulose acetate can be written C₆H_(10−x)O₅(CH₃CO)_(x.);with 39.7% by weight of acetyl groups, the calculation yields x=2.44,thus the formula of this product is C₆H_(7.56)O₅(CH₃CO)_(2.44) with anaverage molar mass of 265.

[0186] Since cellulose acetate is a hydrate of carbon, its reducingpower can thus be calculated by taking nothing into account except thenumber of total carbon atoms, i.e. 10.88 in the product used. With areducing power of 4 electrons per mol of carbon.

[0187] Initially, the mixture contains 4.2 10⁻² mol of celluloseacetate, i.e. 0.459 mol carbon. The final product contains 9.8 10⁻² molcarbon. 0.36 Mol carbon was consumed, which is adequate for explainingthe synthesis of LiFePO₄ under these conditions.

Example 5′ Electrochemical Behavior of the Sample of CarbonatedTriphylite Prepared in Example 5

[0188] The material prepared in example 5 was tested in CR 2032 buttoncells described in example 1″.

[0189]FIG. 9 shows cycles 1 and 10 obtained by slow voltametry. (20mV.h⁻¹) controlled by a battery cycler of the Macpile® type. The twocycles are superimposed, which indicates good cycling capability. Theinitial capacity of this sample is 92% of the theoretical capacity. Thiscapacity is maintained in cycling.

[0190] Even though it leads to samples of LiFePO₄ with good performance,the synthesis taught in example 5 is relatively restricting at thelaboratory level, in spite of its simplicity. In fact, only thecellulose acetate, and at the end of the synthesis the residual carbon,can buffer the atmosphere. Thus it is essential to work with neutralgases purified from all traces of water and oxygen, which is difficultto carry out at the industrial level. A variation in the gas purity or alack of seal integrity in the kiln manifests itself by the obtaining ofan oxidized product.

Example: 6 Preparation of an Iron Phosphosulfate with Nasicon Structure

[0191] In a 500 ml polypropylene flask, 9.4 g of trivalent ironphosphate FePO₄,2H₂O, 7 g trivalent iron sulfate, 4.4 g of ammonium andtitanium oxalate complex (NH₄)₂TiO(C₂O₄)₂, 6.2 g of ammonium sulfate and8.9 g lithium acetate, 1.7 g sucrose and 250 ml methylethylacetone areadded. The mixture is ground on rubber rollers in the presence ofaluminum cylinders (Φ=10 mm, H=10 mm) for 48 hours. The suspension isdried and the powder is ground again in a mortar. The powder obtained isprocessed at 400° in air for 2 hours. After cooling, the calcinationresidue is processed at 670° C. for one hour in a tube kiln in anatmosphere of ammonia in argon (5%), the temperature being raised 5° C.per minute. The gas scavenging is maintained throughout the cooling.

[0192] The gray-black product obtained has the formulaLi_(1.35)Fe_(0.85)Ti_(0.15) SP₀₅O₆ and contains 1.3% carbon conductor.The material tested as an electrode under the conditions in example 1′has a capacity of 115 mAh in the voltage range 3.6-2.5 V.

Example 7 Synthesis Using Non-Commercial Iron Phosphate Prepared fromMetallic Iron

[0193] In this example, carbonated LiFePO₄ was synthesized usingnon-commercial iron phosphate obtained by the action of phosphoric acidon metallic iron.

[0194] The iron powder (325 mesh) was mixed in stoichiometric quantityof an aqueous solution prepared using a commercial solution of 85%phosphoric acid. The mixture was kept under agitation overnight atambient temperature. Bubbling oxygen through makes it possible tooxidize the iron (II) passing in solution into iron (III), whichprecipitates with the phosphate ion. After one night, the solution nolonger contains metallic iron, but rather a slightly grayish powder.Since the solutions of iron (II) are very sensitive to oxidation, themajority of the iron is in oxidation state (III). In certain cases,after dissolving of the metallic iron by bubbling oxygen in asupplementary oxidation was carried out using hydrogen peroxide toinsure that all of the iron is in oxidation state (III). In this case,the powder in solution is slightly yellowish due to traces of the peroxocomplexes.

[0195] The stoichiometric quantity of lithium calculated using thequantity of initial metallic iron was added in the form of lithiumcarbonate, as well as the carbon source (87% hydrolyzed polyvinylalcohol: 20% by weight in comparison to the weight of initial iron)directly in the solution+powder mixture. The grinding is carried out inthis medium. After evaporation of the aqueous solution the mixtures weresubjected to thermal processing described in example 2.

[0196] The products obtained contain 1.1% carbon (determined byelementary analysis). Their electronic conductivity measured asdescribed in example 1′ is 2.10⁻³ S.cm⁻¹. Their electrochemical behaviorcorresponds to carbonated LiFePO₄. 90% of the theoretical capacity isexchanged in a reversible manner in the course of cycling

Example 8 Synthesis Using Non-Commercial Iron Phosphate Prepared fromFe₂O₃

[0197] In this example, carbonated LiFePO₄ was synthesized usingnon-commercial iron phosphate obtained by the action of phosphoric acidon ferric oxide Fe₂O₃. The Fe₂O₃ powder (<5 microns) was mixed instoichiometric quantity of an aqueous solution prepared using acommercial solution of 85% phosphoric acid. The mixture was kept underagitation overnight under reflux. After one night, the mixture containsa pale pink powder. As before the quantity of lithiun carbonaterequired, as well as the carbon source (87% hydrolyzed polyvinylalcohol: 15% by weight in comparison to the weight of initial Fe₂O₃),was added directly to the solution containing the synthesized ironphosphate powder. After grinding, the aqueous solution is evaporated.The mixture is dried before being subjected to the thermal processingdescribed in example 2.

[0198] The products obtained contain 0.7% carbon (determined byelementary analysis). Their electronic conductivity measured asdescribed in example 1′ is 2.10⁻⁵ S.cm⁻¹. Their electrochemical behaviorcorresponds to carbonated LiFePO₄. 85% of the theoretical capacity isexchanged in a reversible manner in the course of cycling.

Example 9 Synthesis of Carbonated LiFePO₄ Using Iron Oxide FeOOH

[0199] Carbonated LiFePO₄ was synthesized by thermal decomposition ofFeOOH (catalyst grade, 30 to 50 mesh) and LiH₂PO₄ in the presence ofhydrogen (8% in argon). In a first period of time, the stoichiometricquantities of the two compounds, as well as the carbon source (sucrose,15% by weight in comparison to the weight of the initial FeOOH) areground together in isopropanol, The solvent is evaporated and themixture is subjected to the thermal processing described in example 2.

[0200] The resulting sample contains 0.8% carbon. Its electronicconductivity measured as described in example 1′ is 6.10⁻⁵ S.cm⁻¹. Itselectrochemical behavior corresponds to carbonated LiFePO₄. 92% of thetheoretical capacity is exchanged in a reversible manner in the courseof cycling.

Example 10 Preparation of LiFe_(0.5)Mn_(0.5)PO₄ in Reducing Atmosphere

[0201] LiFe_(0.5)Mn_(0.5)PO₄ was prepared by mixing stoichiometricquantities of LiH₂PO₄, FeC₂O₄.2H₂O and (CH₃COO)₂Mn.4H₂O. These compoundsare ground in heptane. After drying, the mixture is heated progressivelyto 400° C. in air to decompose the acetate and oxalate groups. Thistemperature is maintained for 8 hours. In the course of this processing,iron (II) oxidizes to iron (III). The mixture is then ground again in anacetone solution containing the carbon precursor (cellulose acetate39.7% by weight of the groups) 5% by weight with respect to themixture). After drying, the mixture is processed thermally with 1:1CO/CO₂ scavenging according to the protocol described in example 3.

[0202] The final compound contains 0.8% carbon. Its electronicconductivity is 5.10⁻⁴ S.cm⁻¹

Example 10 Performance of a Battery Containing the Sample Prepared inExample 10

[0203] The electrochemical behavior of the LiFe_(0.5)Mn_(0.5)PO₄ samplewas evaluated at ambient temperature in a lithium battery containing aliquid electrolyte.

[0204] The cathodes are made up of a mixture of active material, ofcarbon black and of a bonding agent (PVDF in solution inN-methylpyrrolidone) in a ratio of 85:5:10.

[0205] The composite is spread on an aluminum current collector. Afterdrying, the electrodes of 1.3 cm² and with a capacity of around 1.6 mAhare cut with a hollow punch. The batteries are assembled in a glove boxwith inert atmosphere.

[0206] The measurements are carried out in an electrolyte containing 1MLiClO₄ in an EC:DMC mixture 1:1. The anode is made of lithium. The testsare carried out at ambient temperature.

[0207]FIG. 8 presents the charging and discharging curves of a batterycycled in galvanostatic mode between 3 and 4.3 volts. The charging anddischarging loads applied correspond to C/24 (the battery is charged 24hours, then discharged for the same amount of time).

[0208] The discharging curve has two plateaus: the first around 4 Vcorresponds to the reduction of manganese (III) to manganese (II) andthe second, around 3.4 V, corresponds to the reduction of iron (III) toiron (II). The specific capacity obtained during discharge is 157mAh.g⁻¹, which corresponds to 92% of the theoretical capacity.

[0209] The reduction is carried out in the presence of hydrogen (8% inargon).

1. A method for the synthesis of compounds of the formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6 and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals in the first line of the periodic table, M′ is anelement with fixed valency chosen among Mg²⁺, Ca²⁺, Al³⁺, Zn²⁺ or acombination of these same elements and X is chosen among S, P and Si, bybringing into equilibrium, in the required proportions, a mixturecontaining at least: a) a source of M, at least one part of the saidtransition metal or metals that constitute M being in an oxidation stategreater than that of the metal in the final compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n); b) a source of an element M′ ; c) acompound that is a source of lithiun; and d) possibly a compound that isa source of X, the sources of the elements M, M′ , Li and X beingintroduced or not, in whole or in part, in the form of compounds havingmore than one source element, and the synthesis being carried out byreaction and bringing into equilibrium, in the proportions required, themixture (preferably intimate and/or homogeneous mixture) of theprecursors a) to d) and by reduction in such a way as to bring thetransition metal or metals to the desired degree of valency.
 2. A methodof synthesis according to claim 1, in which the reduction of the mixtureof precursors a) to d) is carried out with a reducing gaseousatmosphere.
 3. A method of synthesis according to claim 1 or 2, in whichthe source of M is also the source of X and/or the source of M′ is alsothe source of X and/or the source of lithium is also the source of Xand/or the source of X is also the source of lithium.
 4. A method ofsynthesis according to any one of claims 1 to 3, in which bringing themixture of precursors a) to d) into equilibrium is carried out in theform of an intimate and/or homogeneous mixture.
 5. A method of synthesisaccording to any one of claims 1 to 4, in which the transition metal ormetals is (are) chosen at least partially in the group constituted byiron, manganese, cobalt and nickel, the remainder of the transitionmetals preferably being chosen in the group constituted by vanadium,titanium , chromium and copper.
 6. A method of synthesis according toclaim 5, in which the compound that is the source of M is in a state ofoxidation that can vary from 3 to
 7. 7. A method of synthesis accordingto claim 5, in which the compound that is the source of M is iron (III)oxide or magnetite, manganese dioxide, di-vanadium pentoxide, trivalentiron phosphate, trivalent iron nitrate, trivalent iron sulfate, ironhydroxyphosphate and lithiumn hydroxyphosphate, trivalent iron sulfateor nitrate or a mixture of these.
 8. A method of synthesis according toany one of claims 1 to 7, in which the compound that is the source oflithium is chosen in the group constituted by lithium oxide or lithiumhydroxide, lithium carbonate, the neutral phosphate Li₃PO₄ the acidphosphate LiH₂PO4 the orthosilicates, the metasilicates or thepolysilicates of lithium, lithium sulfate, lithium oxalate and lithiumacetate or a mixture of these.
 9. A method of synthesis according toclaim 8, in which the lithium source compound is lithium carbonate ofthe formula Li₂CO₃.
 10. A method according to any one of claims 1 to 9,in which the source of X is selected in the group constituted bysulfuric acid, lithium sulfate, phosphoric acid and its esters, theneutral phosphate Li₃PO₄ or the acid phosphate LiH₂PO₄, the monoammoniumor diammonium phosphates, trivalent iron phosphate, manganese andammonium phosphate (NH₁MnPO₄), silica, lithium silicates, alkoxysilanesand their partial hydrolysis products and mixtures of the latter.
 11. Amethod of synthesis according to claims 10, in which the X precursorcompound is iron phosphate, preferably iron (III) phosphate, anhydrousor hydrated.
 12. A method according to any one of claims 1 to 11, inwhich at least one of the lithium derivatives obtained is of the formulaLiFePO₄, LiFe_(1−s)Mn_(s)PO₄ wherein 0≦s≦0.9, LiFe_(1−y)Mg_(y)PO₄ andLiFe_(1−y)Ca_(y)PO₄ wherein 0≦y≦0.3, LiFe_(1−s−y)Mn_(s)Mg_(y)PO₄ wherein0≦s≦1 and 0≦y≦0.2, Li_(1+x)FeP_(1−x)Si_(x)O₄ wherein 0≦x≦0.9,Li_(1+x)Fe_(1−s)Mn_(s)P_(1−x)Si_(x)O wherein 0≦s≦1,Li₁₊₂Fe_(1−s−z)Mn_(s)P_(1−z)S_(z)O₄ wherein 0≦s≦1, 0≦z≦0.2,Li_(1+2q)Fe_(1−s−q)Mn_(s)PO₄ where 0≦s≦1, and 0≦q≦0.3,Li_(1+r)Fe_(1−s)Mn_(s)(S_(1−r)P_(r)O₄)_(1.5) wherein 0≦r≦1, 0≦s,t≦1 orLi_(0.5+u)Fe_(1−t)Ti_(t)(PO₄)_(1.5) wherein 0≦t≦1 and wherein 0≦u≦1.5.13. Method of synthesis according to any one of claims 1 to 12, in whichthe compounds of formula Li_(x)M_(1−y)M′_(y)(XO₄)_(n) have an olivine orNasicon structure, including the monoclinic form.
 14. Method ofsynthesis according to any one of claims 1 to 13, in which reduction isobtained by the action of a reducing atmosphere chosen in such a way asto be able to reduce the oxidation state of the metallic ion M to thelevel required for the composition of the compound without reducing itto the neutral metallic state.
 15. Method of synthesis according toclaim 14, in which the reducing atmosphere contains hydrogen or a gasthat is capable of generating hydrogen under the synthesis conditions,ammonia or a substance capable of generating ammonia under the synthesisconditions or carbon monoxide, these gases being used in their purestate or in mixtures and it also being possible to use them in thepresence of water vapor and/or in the presence of carbon dioxide and/orin the presence of a neutral gas (such as nitrogen or argon).
 16. Methodof synthesis according to claim 15, in which the reducing atmosphere ismade of a mixture of CO/CO₂ or H₂/H₂O, NH₃/H₂O or a mixture of them,generating an oxygen equilibrium pressure that is less than or equal tothat determined by the transition metal at the state of oxidationcorresponding to the precursors introduced to form the compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n), but greater than that corresponding to thereduction of any one of the transition elements present to the metallicstate, insuring the thermodynamic stability ofLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in the reaction mixture, independent ofthe time of reaction of the synthesis.
 17. Method of synthesis accordingto claim 15, in which the reducing atmosphere is made up of a mixture ofCO/CO₂, H₂/H₂O, NH₃/H₂O or a mixture of them, generating an oxygenequilibrium pressure that is less than or equal to that determined byone of the transition metals present in Li_(x)M_(1−y)M′_(y)(XO₄)_(n),possibly being able to lead to the reduction of at least this transitionelement to the metallic state, the compound Li_(x)M_(1−y)M′_(y)(XO₄)_(n)being obtained by controlling the temperature and the contact time withthe gaseous phase; the synthesis temperature preferably being between200 and 1200° C., still more preferably between 500 and 800° C. and thetime of contact between the reaction mixture and the gaseous phasepreferably being between 2 minutes and 5 hours and still more preferablybetween 10 and 60 minutes.
 18. Method according to claim 16 or 17, inwhich the gaseous reducing atmosphere is obtained by decomposition, in avacuum or in an inert atmosphere, of an organic compound or of a mixtureof organic compounds containing at least hydrogen and oxygen, boundchemically, and the pyrolysis of which generates carbon monoxide and/ora mixture of carbon dioxide and monoxide, of hydrogen and/or a mixtureof hydrogen and water vapor that is able to carry out the reduction thatleads to the formation of the compound Li_(x)M_(1−y)M′_(y)(XO₄)_(n). 19.Method of synthesis according to claim 16 or 17, in which the gaseousreducing atmosphere is obtained by partial oxidation by oxygen or byair, of a hydrocarbon and/or of carbon called sacrificial carbon,possibly in the presence of water vapor (preferably the water vapor isin an amount comprised between 0.1 and 10 molecules, inclusively, of H₂Oper atom of carbon in the hydrocarbon) at an elevated temperature(preferably comprised between 400 and 1200° C.), making possible theformation of carbon monoxide or hydrogen or of a mixture of carbonmonoxide and hydrogen.
 20. Method of synthesis according to claim 19 inwhich sacrificial carbon is chosen from the group constituted by naturalor artificial graphite, carbon black or acetylene black and cokes(preferably petroleum cokes), the sacrificial carbon preferably being inthe form of particles with a size that is preferably less than 15micrometers, and preferably less than 2 micrometers.
 21. Method ofsynthesis according to claim 18 or 19, in which the quantity ofsacrificial carbon is less than or essentially equal to the quantityrequired to reduce the reaction mixture without allowing any residualsacrificial carbon; this quantity is preferably a carbon atom that isable to combine with an oxygen atom at temperatures greater than 750° C.and it is preferably an atom that can combine with 2 oxygen atoms attemperatures lower than 750° C.
 22. Method of synthesis according toclaim 18 or 19, in which the quantity of sacrificial carbon is equal,mol per mol, to half the quantity of oxygen needed to be removed fromthe mixture of the precursor compounds to obtain the materialLi_(x)M_(1−y)M′_(y)(XO₄)_(n) by reduction, when the reaction is carriedout below 710° C. and equal, mol per mol, to this quantity when thereaction is carried out below this temperature.
 23. Method of synthesisfor a material made of particles, the said particles comprising a coreand a coating and/or being connected to each other by cross-linking, thesaid core comprising at least one compound of the formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals from the first line of the periodic table, M′ is anelement with fixed valency chosen from among Mg²⁺, Ca²⁺, Al³⁺, Zn²⁺ andX is chosen from among S, P and Si, the said coating being made ofcarbon connecting at least two particles to each other, and the saidcross-linking being made of carbon connecting at least two particles toeach other, the said method consisting of bringing into equilibrium, inthe proportions required, a mixture (preferably intimate and/orhomogeneous) containing at least a) a source of M, at least one part ofthe said transition metal or metals that constitute M being in anoxidation state greater than that of the metal in the final compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n); b) a source of an element M′; c) acompound that is a source of lithium; and d) possibly a compound that isa source of X; the sources of the elements M, M′, Li and X beingintroduced or not in whole or in part, in the form of compounds havingmore than one source element, and the synthesis being carried out byreaction and bringing into equilibrium, in the proportions required, themixture of the precursors a) to d) with a reducing gaseous atmosphere,in such a way as to bring the transition metal or metals to the desireddegree of valency, the synthesis being carried out in the presence of acarbon source called carbon conductor, the synthesis thus leading to thesaid material, by a pyrolysis step for the carbon source compound after,or preferably simultaneously with, the steps of preparation of themixture of the precursors and reduction of the mixture obtained.
 24. Amethod of synthesis according to claim 23, in which the carbon presentin the material, in the form of coating and cross-linking, adheresintimately to the particles of the material and lends to the latter anelectronic conductivity that is greater than that of the materialconstituted by the corresponding non-coated particles.
 25. A method ofsynthesis according to claim 23 or 24, characterized in that theaddition of carbon conductor is carried out after the synthesis ofLi_(x)M_(1−y)M′_(y)(XO₄)_(n).
 26. A method of synthesis according toclaim 23, characterized in that the addition of carbon conductor iscarried out simultaneously with the synthesis ofLi_(x)M_(1−y)M′_(y)(XO₄)_(n).
 27. A method of synthesis according to anyone of claims 23 to 26, in which the reaction parameters, in particularthe kinetics of the reduction by the gaseous phase, are chosen in such away that the carbon conductor does not participate in a significant wayin the reduction process.
 28. A method of synthesis according to claim27, in which the reaction parameters, such as the flow and thecomposition of the gaseous phase, temperature and contact time, arechosen in such a way that the carbon conductor does not participate in asignificant way in the reduction process, i.e. the reduction process isdue to the gaseous phase, and in particular in such a way that thereaction temperature is preferably less than 900° C. and the reactiontime less than 5 hours, in a manner that is eaten more advantageous ifthe reaction temperature is below 800° C. and/or for times less than 1hour.
 29. A method of synthesis according to claim 1 or 23, in which thevalue of x in Li_(x)M_(1−y)M′_(y)(XO₄)_(n) is chosen in such a way as toinsure thermodynamic control and/or rapid kinetics of the reduction bymaking it possible to select reducing gaseous atmospheres that areeasily accessible by simple mixture of gases or by reforming of simpleorganic molecules.
 30. A method of synthesis according to any one ofclaims 23 to 29, in which the organic substance that is the source ofcarbon conductor is selected in such a way that the particles ofmaterial obtained after the pyrolysis step essentially have the form andgranulometric distribution of the precursors of the synthesis reaction.31. A method of synthesis according to any one of claims 23 to 30, inwhich the organic substance that is the source of the carbon conductoris selected in the the group constituted by polymers and oligomerscontaining a carbon skeleton, simple carbohydrates or polymers and thearomatic hydrocarbons.
 32. A method of synthesis according to claim 31,in which the organic substance that is the source of carbon conductor ischosen in such a way as to leave a deposit of carbon conductor on thesurface (coating) of the solid particles that constitute the materialand/or between these solid particles, carbon bridges (cross-linking), atthe time of pyrolysis.
 33. A method of synthesis according to any one ofclaims 23 to 32, in which the carbon conductor source contains, in thesame compound or in the mixture that constitutes this source, oxygen andhydrogen that are bound chemically and from which pyrolysis locallyreleases carbon monoxide and/or carbon dioxide and/or hydrogen and watervapor that contributes, in addition to depositing carbon, to creatinglocally the reducing atmosphere required for synthesis of the materialLi_(x)M_(1−y)M′_(y)(XO₄)_(n).
 34. A method of synthesis according toclaim 32 or 33, in which the organic substance that is the source ofcarbon conductor is at least one of the compounds of the groupconstituted by polyethylene, polypropylene, glucose, fructose, sucrose,xylose, sorbose, starch, cellulose and its esters, block polymers ofethylene and ethylene oxide and polymers of furfuryl alcohol.
 35. Amethod of synthesis according to any one of claims 23 to 34, in whichthe source of carbon conductor is added at the start of, or in thecourse of, the mixing step of the reaction precursors a) to d), such asdefined in claim
 1. 36. A method of synthesis according to claim 35, inwhich the amount of substance that is the source of carbon conductor,present in the reaction medium subjected to reduction, is chosen suchthat the amount of carbon conductor in the reaction medium is preferablybetween 0.1 and 25%, inclusively, and still more preferably between 0.3and 1.5%, inclusively, of the total mass of the reaction mixture.
 37. Amethod according to any one of claims 1 to 36, in which the thermalprocessing (which includes the formation reaction ofLi_(x)M_(1−y)M′_(y)(XO₄)_(n) and the reduction and pyrolysis) is carriedout by heating from normal temperature to a temperature comprisedbetween 500 and 1100° C.
 38. A method of synthesis according to claim37, in which the maximum temperature reached is comprised between 500and 800° C.
 39. A method of synthesis according to any one of claims 1to 38, in which the temperature and duration of the synthesis are chosenas a function of the nature of the transition metal, i.e. above aminimum temperature at which the reactive atmosphere is capable ofreducing the transition element or elements to their oxidation staterequired in the compound Li_(x)M_(1−y)M′_(y)(XO₄)_(n) and below atemperature or a time leading to a reduction of the transition elementor elements to the metallic state or an oxidation of the carbonresulting from pyrolysis of the organic substance.
 40. A method ofsynthesis according to claim 39, in which the compoundLi_(x)M_(1−y)M′_(y)(XO₄)_(n) is LiMPO₄ and in which the amount of carbonconductor after pyrolysis is comprised between 0.1 and 10% bed mass incomparison to the mass of the compound LiMPO₄.
 41. A method of synthesisaccording to any one of claims 23 to 40, in which the compound that isthe source of carbon is easily dispersible at the time of the processingused to insure an intimate mixture of precursors a) to d) by agitatingand/or mechanical grinding and/or by ultrasonic homogenizing in thepresence, or not, of a liquid or by spray-drying of a solution of one ormore precursors and/or of a suspension or of an emulsion.
 42. A methodof synthesis according to any one of claims 23 to 41, comprising twosteps: i) intimate grinding, dry or in a solvent, of the sourcecompounds including carbon conductor, and drying if necessary; and ii)thermal processing with scavenging by a controlled reducing atmosphere.43. A method of synthesis according to any one of claims 23 to 42,characterized in that the material obtained has a conductivity that isgreater than 10⁻⁸ S.cm⁻¹, when measured on a sample of powder compactedat 100 Kg.cm⁻².
 44. A method of synthesis according to any once ofclaims 1 to 22, in which the compound obtained has the formula LiFePO₄.45. A method of synthesis according to any one of claims 23 to 44, inwhich the core of the particles of the material obtained is essentially(preferably at least for 95%) made of a compound of formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n) (preferably of formula LiFePO₄) and theamount of carbon conductor for coating and/or cross-linking, in thematerial obtained, is preferably comprised between 0.2 and 5%,inclusively, still more preferably between 0.3 and 3%, in comparison tothe mass of material obtained at the time of synthesis.
 46. A method ofsynthesis according to claim 44 or 45, in which the compound that is thesource of iron is chosen at least partially from the group made up ofiron phosphates, iron oxyphosphates or hydroxyphosphates, iron oxidesand lithium oxides, in which at least a part of the iron is in theoxidation state III, as well as mixtures of the latter.
 47. A method ofsynthesis according to any one of claims 44 to 46, in which the compoundthat is the source of lithium is lithiun phosphate, lithiumdihydrogenophosphate, lithium carbonate, lithium acetate or lithiumhydroxide, as well as mixtures of the latter.
 48. A method of synthesisaccording to any one of claims 44 to 47, in which the compound that isthe source of phosphorus is an ammonium phosphate, orthophosphoric,metaphosphoric or pyrophosphoric acid or phosphorus pentoxide.
 49. Amethod of synthesis according to any one of claims 1 to 48, in which thereducing step is carried out in the reactor that is used or was used forpreparation of the mixture of precursors or in a different reactor(preferably in a reformer).
 50. Particles of a compound of formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals from the first line of the periodic table, M′ is anelement with fixed valency selected in the group constituted by Mg²⁺,Ca²⁺, Al³⁺, Zn²⁺ and X is selected in the group constituted by S, P andSi, the said particles having a size comprised between 0.05 micrometersand 15 micrometers, preferably between 0.1 and 10 micrometers and thesaid particles having a conductivity that is greater than 10⁻⁸ S.cm⁻¹,measured on a sample of powder compressed at a pressure greater than orequal to 3000 and preferably at 7350 Kg.cm⁻².
 51. Material in the formof particles comprising a core and/or a coating of carbon around thecore and/or a cross-linking of carbon among the particles, the said corecontaining at least one compound of the formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals from the first period of the periodic table, M′ is anelement with fixed valency chosen from among Mg²⁺, Ca²⁺, Al³⁺, Zn²⁺ andX is chosen from among S, P and Si, the said material having aconductivity greater than 10⁻⁸ S.cm⁻¹, measured on a sample of powdercompressed at a pressure greater than or equal to 3000 Kg.cm⁻², andpreferably at 7350 Kg.cm⁻².
 52. Material that can be obtained accordingto a method according to any one of claims 23 to 49, comprising a coreand a coating and/or a cross-linking, the said material having a totalcarbon amount comprised between 0.1 and 0.2%, inclusively, and that ispreferably greater than 0.1% of the total mass of the material. 53.Material in the form of particles comprising a core and/or a coating ofcarbon around the core and/or a cross-linking of carbon among theparticles, the said core containing at least one compound of the formulaLi_(x)M_(1−y)M′_(y)(XO₄)_(n), in which x, y and n are numbers such as0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture oftransition metals from the first period of the periodic table, M′ is anelement with fixed valency chosen from among Mg²⁺, Ca²⁺, Al³⁺, Zn²⁺ andX is chosen from among S, P and Si, the said material having aconductivity greater than 10⁻⁸ S.cm⁻¹, measured on a sample of powdercompressed at a pressure greater than 3000 Kg.cm⁻², preferably at 7350Kg.cm⁻², and having a total carbon amount greater than 0.3% of the totalmass of the material.
 54. Electrochemical cell comprising at least twoelectrodes and at least one electrolyte, characterized in that at leastone of its electrodes contains at least one of the materials accordingto any one of claims 50 to
 53. 55. Cell according to claim 54,characterized in that the electrolyte is a polymer, solvating or not,optionally plasticized or gelled by a polar liquid containing one ormore metallic salts in solution.
 56. Cell according to claim 54,characterized in that the electrolyte is a polar liquid immobilized in aMicroporous separator and containing one or more metallic salts insolution.
 57. Cell according to claim 56, in which at least one of themetallic salts is a lithium salt.
 58. Cell according to any one ofclaims 54 to 57, characterized in that at least one of the negativeelectrodes is made of metallic lithium, a lithium alloy, especially withaluminum, antimony, zinc, tin, possibly in nanometric mixture withlithium oxide or a carbon insertion compound, especially graphite, adouble nitride of lithium and iron, cobalt or manganese, a lithiumtitanate of the formula Li_(x)Ti_((5+3y)/4)O₄, wherein 1≦x≦(11-3y)/4(or) wherein 0≦y≦1.
 59. Cell according to any one of claims 53 to 58,characterized in that at least one of the positive electrodes containsone of the products obtainable by a procedure according to any one ofclaims 1 to 49, used alone or in mixture with a double oxide of cobaltand lithium, or with an complex oxide of the formulaLi_(x)Ni_(1−y−z−q−r)Co_(y)Mg_(z)Al_(r)O₂ wherein 0.05≦x≦1, 0≦y, z andr≦0.3, or with an complex oxide of the formulaLi_(x)Mn_(1−y−z−q−r)Co_(y)Mg_(z)AlrO₂−qF_(q) wherein 0.05≦x≦1 and 0≦y,z, r, q≦0.3.
 60. Cell according to any one of claims 53 to 59,characterized in that the polymer used to bond the electrodes or used aselectrolytes is a polyether, a polyester, a polymer based on methylmethacrylate units, an acrylonitrile-based polymer and/or a vinylidenefluoride or a mixture of the latter.
 61. Cell according to any one ofclaims 53 to 60, characterized in that the cell contains anon-protogenic solvent that preferably contains ethylene or propylenecarbonate, an alkyl carbonate having 1 to 4 carbon atoms,γ-butyrolactone, a tetraalkylsulfamide, an α-ω dialkylether of a mono-,di-, tri-, tetra- or oligo-ethylene glycol with molecular weight lessthan or equal to 5000, as well as mixtures of the above-named solvents.62. Cell according to any one of claims 53 to 61, characterized in thatit functions as a primary or secondary generator, as a supercapacity oras a light modulation system.
 63. Electrochemical cell according to anyone of claims 53 to 62, characterized in that it functions as asupercapacity, characterized in that the positive electrode material isa material according to claims 50 to 52, and the negative electrode is acarbon with a specific surface area greater than 50 m².g⁻¹ in the formof powder, fiber or mesoporous composite of a carbon-carbon compositetype.
 64. Electrochemical cell according to any one of claims 53 to 63,characterized in that it functions as a light modulation system and inthis case, the optically inactive counter-electrode is a materialaccording to claims 50 to 53, spread in a thin layer on a transparentconductor support of a glass or polymer type covered with doped tinoxide (SnO₂:Sb or SnO₂:F) or doped indium oxide (In₂O₃:Sn).